A flush joint tubular connection has inner and outer diameters substantially the same as the tubing joints which the connection connects. A flush joint tubular connection made by the Hydril Company and covered by numerous patents comprise a first straight thread, a second straight thread of sufficient diameter to pass within the bore of the first thread and a tapered mating seat between the two joints of tubing which is a premium joint of high cost and according to published data, enjoys only 42% axial strength with regard to the pipe wall.
Standard A.P.I. non-upset tubing connections comprise couplings having outer diameters considerably larger than the pipe outer diameter but still only enjoy approximately 42% efficiency as above. A.P.I. does list a "turned down" collar outer diameter to increase clearance between strings, however, the "turned down" diameter still exceeds substantially, the pipe outer diameter.
No prior art discloses a flush joint tubular connection having tapered threads, that when properly assembled, effects optimum stresses within the small end of the external thread and within the large end of the internal thread so as to provide a connection of maximum efficiency. Conventional pipe connections have threads with like tapers and result in a constant diametrical interference along the taper between the external and internal threads, thereby causing excessive stresses or requiring increased wall thickness at the end of the pipe. Excessive stresses reduce the joint strength and an increased wall thickness rules out a flush joint connection.
It is therefore clear that a flush joint connection having a high efficiency as provided by the instant invention is needed for use within oilwells and other pipe assemblies wherein radial clearance is limited.
Standard pipe threads as well as A.P.I. threaded connections have such a tendency to cross-thread that "stabbing guides" are often used at a considerable cost of time and expense. Such threads have an extremely shallow stab depth and a relatively large thread depth, both of which add to the cross-thread problem. Perfect alignment is difficult to attain under normal field conditions and often impossible to attain under difficult conditions. Premium connections such as disclosed by Stone in U.S. Pat. No. 1,932,427 require even closer alignment to stab because of the close fit of straight threads and the "pin-nose" seal 32, which is highly susceptible to damage. To applicants belief no prior art comprised the combination of a deep stab, thread height and thread diameter as required to provide a tapered threaded connection that will stab easily and quickly without the possibility of cross-threading. By way of an example, a 23/8 EU 8rd A.P.I. tubing thread has a 2.473" pin end diameter and a 2.437" box bore at the first thread which allows no entry of the pin into the box at stab position. The counterbore of the box allows entry of the pin only 0.446" affording at best, axial alignment but no angular alignment so less than six degrees of angular misalignment will allow it to cross-thread.
About 1940, A.P.I. changed from 10 V threads to 8rd and a substantial improvement resulted because less gauling occurred during makeup of the threads. It was then commonly assumed "that any thread finer than 8 threads per inch would gall and cross-thread" and that myth persists today. However, the improvement resulted almost entirely from the better thread form, eliminating the sharp edge V threads. The present invention with threads as fine as 20 per inch, run fast and smooth without cross-threading, and it has other features as well.
Conventional "near-flush" connections mentioned above, have two-step straight box threads formed within swaged-out ends and pin threads formed on swaged-in ends. Such swaged ends comprise a single tapered zone extending axially from the pipe body of original pipe diameters having a mean conical angle of taper of approximately two degrees. Typically, such swaged connections are rated by their suppliers as having from 50% to 75% efficiency depending on wall thickness, and with a variety of fluid pressure ratings. Such a swaged connection when compared to a 42% conventional flush joint connection, has improved strength, but at the expense of clearance.
To applicants best knowledge and belief, all such swaged connections now on the market are swaged to form only the degree of taper that approximates the lay of threads to be formed thereto. Typically, before a thread is machined in the tapered zone, a clean-up cut is made to assure there being enough metal to fully form the threads. Unfortunately, such a cut reduces the cross-section area of the tapered zone which limits connection efficiency. Additionally, production machining allows for only approximate axial positioning of the pipe in the machine prior to gripping the pipe in the chuck and such approximation can cause further thinning of the tapered zone. Thirdly, if first measurement of a freshly cut thread indicates that a thread recut is required, then the swage must be cut off and the end reswaged before even a 75% thread could be cut at that end. Therefore, in addition to the basic disadvantages of a two-step thread having a pin-nose seal, it is now even more clear why suppliers of pipe threads that are formed on swaged ends cannot provide a family of pipe connections with efficiencies greater than 75%.
Applicants U.S. Pat. No. 4,813,717 which is in the line of priority for the present application, discloses a connection with selective efficiency between 75% and 100% for non-upset pipe using a coupling in one embodiment per claims 1-17 and an integral connection in another embodiment per claims 18-19. The present invention is complimentary to said patent and teaches configurations for connections having swaged ends. To applicants best knowledge and belief, no non-upset integral connection is currently available that will meet the strain design criteria above. For users who prefer integral non-upset pipe connections, there is clearly a need for one with an efficiency sufficient to meet the strain design criteria defined above.